Plants and Biological Filtration

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Plants are much more than tank decorations; they help keep the fish healthy. Nitrogenous compounds, particularly ammonia and nitrite, are extremely toxic to fish. Hobbyists have for many years relied heavily on the bacterial process of nitrification (i.e., ‘biological filtration’) to convert these toxic compounds into non-toxic nitrates. Hobbyists and even retailer of aquatic plants too easily ignore nitrogen uptake by aquarium plants or assume (incorrectly) that aquarium plants mainly take up nitrates.

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Aquatic Plants Prefer Ammonium Over Nitrates

Many terrestrial plants like peas and tomatoes do grow better with nitrates than ammonium [1]. Thus, some botanists assumed that aquatic plants would similarly take up and grow better with nitrates. However, actual experimental studies suggest otherwise.

Scientists from all over the world have studied nitrogen uptake in aquatic plants under a variety of experimental conditions. I was able to locate published studies on 33 different aquatic plant species. Only 4 of the 33 species preferred nitrates (Table 1).

Table 1. Nitrogen Preference of Tested Species. Full references for these studies are listed elsewhere [2]

Ammonium:

Nitrate:

Agrostis canina

Echinodorus ranunculoides

Callitriche hamulata

Littorella uniflora

Ceratophyllum demersum

Lobelia dortmanna

Drepanocladus fluitans

Luronium natans

Eichhornia crassipes

Elodea densa

Elodea nuttallii

Fontinalis antipyretica

Hydrocotyle umbellata

Juncus bulbosus

Jungermannia vulcanicola

Lemna gibba

Lemna minor

Marchantia polymorpha

Myriophyllum spicatum

Pistia stratiotes

Ranunculus fluitans

Salvinia molesta

Scapania undulata

Sphagnum cuspidatum

Sphagnum fallax

Sphagnum flexuosum

Sphagnum fuscum

Sphagnum magellanicum

Sphagnum papillosum

Sphagnum pulchrum

Sphagnum rubellum

Spirodela oligorrhiza

Zostera marina

Even then, these 4 species come from unusually nutrient-deprived environments that are not typical for aquarium plants. Moreover, the extent of the ammonium preference is monumental. For example, the duckweedLemna gibba removed 50% of the ammonium in a nutrient solution within 5 hours, even though the solution contained over a hundred times more nitrates than ammonium [3]. Elodea nuttallii, placed in a mixture of ammonium and nitrates, removed 75% of the ammonium within 16 hours while leaving the nitrates virtually untouched (Fig 1). Only when the ammonium was gone, did it seriously take up nitrates. Likewise, when the giant duckweed Spirodela oligorrhiza was grown in media containing a mixture of ammonium and nitrate, the ammonium was rapidly taken up whereas the nitrates were virtually ignored (Fig 2).

Because the plants for this particular study were grown under sterile conditions, the ammonium removal could not have been due to nitrification. Also, the investigator showed that plants grew rapidly during the study confirming that the ammonium uptake was not an experimental artefact, but that it probably accompanied the increased plant biomass and need for nitrogen. (The N concentration in aquatic plants ranges from 0.6 to 4.3% of the their dry weight [4].

Table 2 shows how fast nitrate and ammonium is removed from the water by the water lettuce (Pistia stratiotes). Plants placed in nutrient solution containing 0.025 mg/l of nitrate-N required 18 hours to take up the nitrates. However, similar plants placed in nutrient solution containing 0.025 mg/l of ammonium-N required only 3.9 hours to take up the ammonium. When the investigators increased the nitrogen concentration, the difference was even greater. Thus, at 13 mg/l N, plants required 71 hours (almost 3 days) to take up nitrate, but if the N was supplied as ammonium, uptake was still just 4 hours.

Nitrate uptake seems to require more effort for aquatic plants than ammonium. For example, the water lettuce took up nitrates much slower in the dark [5], while ammonium uptake was the same in the light or the dark. This suggests that nitrate uptake requires more energy than ammonium uptake. Furthermore, nitrate uptake often has to be induced before it can be measured. For example, maximum nitrate uptake in the water lettuce did not occur until after the plants had been acclimated to pure nitrates for 24 hours (any ammonium in the water would have prevented nitrate uptake).

Investigators placed plants in beakers with nutrient solution that contained increasing amounts of N given to plants as either pure nitrates or pure ammonium. Hours required for N removal are based on the assumptions that there is 1 gram of plant dry weight per litre and that the solution is constantly stirred. (Note: mg/l = milligrams per litre.)

Table 2. Hours Required for Water Lettuce to Take Up Nitrates v. Ammonium [5].

Nitrogen in the Nutrient Solution

Nitrate Uptake

Ammonium Uptake

mg/l

Hours

Hours

0.025

18

3.9

0.05

18

4.1

0.1

19

4.2

0.2

19

4.2

0.4

20

4.2

0.8

21

4.2

1.6

25

4.2

3.2

31

4.3

6.4

44

4.3

13

71

4.3

26

123

4.3

Ammonium actually inhibits nitrate uptake and assimilation in a variety of organisms such as plants, algae, and fungi [6].

For example, algae doesn't take up nitrates if the ammonium concentration is more than about 0.02 mg/l [7]. Nitrate uptake by duckweed promptly ceases when ammonium is added to nutrient solutions [8]. The inhibition is typically reversible, because plants will start to take up nitrates a day or two after all ammonium is removed from the water. One could hypothesize that the ammonium inhibition of nitrate uptake may protect the plant from taking up nitrates, which can drain energy from the plant (see ‘Aquatic Plants versus Biological Filtration’ below).

Nitrite Uptake by Plants

Although plants can use nitrite as an N source, the pertinent question for hobbyists is - Do aquatic plants remove the toxic nitrite before the non-toxic nitrate?
I could not find enough studies in the scientific literature to state conclusively that they do. However, the chemical reduction of nitrites to ammonium requires less of the plant’s energy than the chemical reduction of nitrates to ammonium. (A plant must convert both nitrites and nitrates to ammonium before it can use them to make its proteins.) Thus, it is not surprising that when Spirodela oligorrhiza was grown in media containing both nitrate and nitrite, it preferred nitrite (Fig. 3).

Aquatic Plants Prefer Leaf Uptake of Ammonium

If aquatic plants preferred to get ammonium by root uptake from the substrate rather than leaf uptake from the water, their ability to remove toxic ammonia from the water and protect our aquarium fish would be questionable. Fortunately for hobbyists, aquatic plants seem to prefer leaf uptake of ammonium as opposed to sediment uptake [2]. For example, in a split-chamber experiment with the marine eelgrass (Zostera marina), when ammonium was added to the leaf/stem compartment, root uptake was reduced by 77%. However, when ammonium was added to the root compartment, leaf uptake was not reduced. (In split-chamber experiments, plants are grown with their roots in a sealed bottom compartment and with their stems/leaves in a separate upper compartment.)

Work with other plant species supports the above findings. Apparently, the seagrass Amphibolis antarctica can take up ammonium 5 to 38 faster by the leaves than the roots. And Myriophyllum spicatum planted in fertile sediment grew fine without any ammonium in the water. However, if ammonium was added to the water (0.1 mg/l N), plants took up more N from the water than the sediment.

Several aquatic plants (Juncus bulbosus, Sphagnum flexuosum, Agrostis canina, and Drepanocladus fluitans) were found to take up 71 to 82% of the ammonium from the leaves; their roots took up only a minor amount.

Hobbyists using fertilizer tablets for aquatic plants might want to carefully consider the aquatic plant preference for leaf uptake of ammonium (as opposed to root uptake). In ponds and aquariums, plants should be able to fulfil their N needs from fish-generated ammonium in the water. What’s more; nitrogen added to substrates can be detrimental. Ammonium can be toxic to plant roots. Even nitrates added in substrate fertilizer tablets can create problems. This is because bacteria in the substrate quickly convert nitrates to toxic nitrites.[2]

Aquatic Plants versus Biological Filtration

Plants, algae, and all photo synthesizing organisms use the nitrogen from ammonia - not nitrates - to produce their proteins. If the plant takes up nitrate, it must first be converted to ammonium in an energy-requiring process called ‘nitrate reduction’.

Nitrate reduction in plants appears to be the mirror image of the bacterial process of nitrification. Nitrifying bacteria gain the energy they need for their life processes solely from oxidizing ammonium to nitrates; the total energy gain from the two-steps of nitrification is 84 Kcal/mol. The overall reaction for nitrification is:

NH4+ + 2 O2 >> NO3- + H2O + 2 H+

Plants theoretically must expend essentially the same amount of energy (83 Kcal/mol) to convert nitrates back to ammonium in the two-step process of nitrate reduction The overall reaction for nitrate reduction is:

NO3- + H2O + 2 H+ >> NH4+ + 2 O2

The energy required for nitrate reduction is equivalent to 23.4% of the energy obtained from glucose combustion [1]. Thus, if nitrifying bacteria in biological filters convert all available ammonium to nitrates, plants will be forced—at an energy cost—to convert all the nitrates back to ammonium. This may explain why several aquatic plants (e.g., water hyacinth, Salivinia molesta, hornwort, and Elodea nuttallii) seem to grow better with ammonium or an ammonium/nitrate mixture than when they are forced to grow with pure nitrates [2]. The nitrogen cycle is often presented incorrectly to hobbyists as nitrifying bacteria converting ammonium to nitrates and then plants taking up nitrates. Actually, it consists of both plants and bacteria competing for ammonium. Only if plants are forced to, will they take up nitrates. Thus, nitrates may accumulate even in planted ponds and aquariums.

Nitrification enhanced by filters is essential for protecting fish from toxic ammonia in aquariums without plants. However, planted aquaria are a whole different ballgame. In fact, plants provide an enormously increased surface area within the aquarium for nitrifying bacteria. Planted areas (as opposed to unplanted areas) in natural habitats (rivers, lakes, etc) have been shown to provide an exponentially increased number of colonization sites for bacteria [9]. You can be sure that every leaf and stem surface in an established aquarium is coated with a layer of nitrifying (and other) bacteria.

I have been surprised at how little biological filtration is actually required in my planted aquaria. When I gradually decreased biological filtration by removing the packing media from the canister filters, the fish continued to do well. Finally, years later I took the decisive step and removed the canister and outside filters altogether and just used cheap internal pumps to circulate the water. Fish never missed a beat; the planted tank itself is a filter!

Aquatic plants, then, are much more than ornaments or aquascaping tools. They remove ammonia from the water. Furthermore, they remove it within hours (Fig 1, Table 2). When setting up a planted tank, there is no need to wait 8 weeks to prevent ‘new tank syndrome’. (Nitrifying bacteria require several weeks to establish themselves in new tanks and make biological filtration fully functional.) Thus, I have several times set up a new tank with plants and fish all on the same day.

In summary, there is considerable experimental evidence in the scientific literature showing that aquatic plants vastly prefer ammonium over nitrates as their N source. Even in the presence of abundant nitrates, aquatic plants will be sifting the water 24 hours a day for ammonium. Plants in aquariums also increase ammonium removal by simply increasing colonization sites for nitrifying bacteria. I hope this explains why (in terms of fish health) it is worth the trouble to keep plants in aquariums.

[Much of this article was excerpted from Ecology of the Planted Aquarium by Diana Walstad. The book is readily available from Internet book sellers such as Amazon.com.]